The present invention relates generally to a method for frequency synchronization of a mobile station in a radio communications systems (in particular, a TDMA/CDMA radio communications system), and, more specifically to a mobile station and to a base station which can carry out such method.
The design of digital radio communications systems is shown in J. Oudelaar, xe2x80x9cEvolution towards UMTSxe2x80x9d, PIMRC 94, 5th IEEE International Symp. on Personal, Indoor and Mobile Radio Communications, The Hague, NL, Sep. 18-22 1994, pages 852-856, and M. Lenti, H. Hageman, xe2x80x9cPaging in UMTSxe2x80x9d, RACE Mobile Telecommunications Workshop, Vol. 1, Amsterdam, NL, May 17-19 1994, pages 405-410.
The presently known mobile radio system GSM (Global System for Mobile Communications) is a radio communications system with a TDMA component for subscriber separation (time division multiple access). User information from the subscriber connections is transmitted in time slots in accordance with a frame structure. The transmission is performed in blocks. Furthermore, frequency channels (FCCH frequency correction channels) matched to the timing pattern of the frame structure and serving the purpose of frequency synchronization for the mobile stations are known in the downward direction from the GSM mobile radio systems. In this frequency channel, a mobile station can evaluate a sinusoidal carrier for the purpose of self-synchronization. Frequency synchronization by means of a sinusoidal carrier is disclosed, for example, in WO 91 10305 A.
Time synchronization of time slots in a TDMA radio communications system is disclosed in EP 0 318 684. D1 discloses a method for time synchronization of a mobile station. The radio transmission channel has a signal delay between the base station and mobile station which is proportional to the distance between the mobile station and the base station. In addition, the signal delay can vary strongly in time because of reflections. The signals received by the mobile station are delayed temporally with different severity by the signal delay. This delaying of the signal between the transmitted signal and the received signal is denoted as phase error. The compensation of this delaying of the signal is the subject matter of the method for time synchronization. In such case, the delaying of the signal is constant within one TDMA time slot.
DE 195 49 148.3 discloses a mobile communications system which uses a TDMA/CDMA subscriber separation (CDMA code division multiple access), and applies at the receiving end a JD method (joint detection) in order to undertaken with knowledge of sequence spread codes of a plurality of subscribers to, improved detection of the transmitted user information. Information from a plurality of user data connections which can be distinguished by their spread code are transmitted simultaneously in one frequency channel (TCH traffic channel). However, the division of specific frequency channels for synchronization purposes results in a large loss in capacity in comparison with the GSM system since the frequency range used for a frequency channel is a more broadband one.
It is an object of the present invention, therefore, to provide a method and devices which permit frequency synchronization in conjunction with low consumption of radio resources in a radio communications system.
A radio communications system provides frequency channels which are formed by time slots and by broadband frequency ranges, and in which information from several connections is transmitted simultaneously between mobile stations and a base station; it being possible to distinguish the information from different connections in accordance with a connection-specific fine structure. The different connections also can be formed by a plurality of codes which are assigned to a single mobile station.
According to the present invention, frequency channels in which a symbol sequence is transmitted are temporally repeatedly provided for frequency synchronization for the mobile stations in the downward direction. From received signals, the mobile station to be synchronized determines estimated values for the symbol sequence and compares them with a reference sequence. Consequently, a phase drift in the estimated values with reference to the reference sequence can be calculated. A frequency offset to which is used for frequency synchronization is determined from the phase drift.
By virtue of the fact that a phase drift can be calculated in accordance with the comparison, the possibility arises of simply determining the frequency offset of the mobile station to be synchronized with reference to the carrier of the frequency channel. A sufficiently large and appropriately distributed number of samples is sufficient for the comparison. It is thereby possible for the frequency synchronization to be combined with other measures for synchronizing the mobile station so that the use of radio resources remains low.
Advantageously, the symbol sequence is transmitted in addition to information from further connections. This also means that the radio resources of the air interface between the base station and mobile stations can be better used. Because information can be distinguished in accordance with an impressed fine structure, a time slot is not blocked solely by the synchronization. Rather, such time slot can be used multifariously. The further connections are, in this case, user data connections or signaling connections.
In an embodiment of the present invention, the symbol sequence is transmitted in such a way that at least two estimated values are obtained at a temporal spacing sufficiently large for the phase drift calculation. It is, therefore, not necessary for the symbol sequence to be a continuous sequence. There can be individual symbols or groups of symbols within further known or unknown symbols to be transmitted. The symbols can last over the time period of a time slot, or they can be transmitted in addition to training sequences and/or symbols for other tuning purposes.
The symbol sequence is advantageously spread with the aid of an individual spread code, it being possible for the information on the presence of a symbol sequence to be included in the spread code for time synchronization. The symbol sequence also can be interpreted as a sequence of chips which generate a bandwidth which fills up the frequency range. An additional outlay on processing at the receiver is eliminated by virtue of the fact that the message block with the data for time synchronization can be processed together with the remaining message blocks by despreading.
In accordance with a further embodiment of the present invention, the phase drift is calculated as a proportionality factor in accordance with the method of least error squares from the comparison of the estimated values with the reference sequence. A linear relationship is presupposed between the estimated values and reference sequence; that is, an approximately constant frequency offset. This obtains in the case of the high accuracies of the frequency normal in the mobile station.
It is advantageous to use at least parts of the symbol sequence both for the time synchronization and for the frequency synchronization. Thus, it is possible with the aid of a symbol sequence, for example by correlation, to produce the temporal reference of the transmission within a time slot and, in addition, the frequency reference by determining the phase drift. Only a low network capacity need therefore be set aside for synchronization.